In this study, chronic bipolar electrical stimulating electrodes are implanted into the midbrains of rats. Implantation target sites are the dorsal midbrain tegmentum (DMT) or the ventral midbrain tegmentum (VMT). DMT stimulation produces fearlike, escape-seeking behavior, whereas VMT stimulation produces sterotyped circling behavior. Both types of midbrain stimulation are aversive in that rats stimulated at either site will learn to bar press for escape in the decremental bar-pressing paradigm. In this paradigm, each bar-press decreases the amount of stimulation current by a predetermined per cent of the initial current level. Following acquisition of stable bar-pressing behavior, the animals are given various drugs which modify biogenic amine biochemistry in the central nervous system. BIBLIOGRAPHIC REFERENCES: Kiser, R.S. and R.M. Lebovitz. Monoaminergic mechanisms in aversive brain stimulation. Physiology and Behavior 15: 47-53, 1975.